The long-term goal of this research is to understand the regulation of tissue-specific gene expression in mammalian cells. Using cultured hepatoma cells, hepatoma hybrids, and somatic variants as genetic tools, mechanisms that control expression of specific liver genes in hepatic and non-hepatic cells will be defined. Three main areas of research will be pursued. First, genetic experiments will be performed to dissect the extinction phenotypes of intertypic hepatoma hybrids. Hepatoma x fibroblast hybrids and microcell hybrids will be used to define genetic factors involved in the extinction process, and both monogenic and polygenic extinction phenotypes will be explored. In other experiments, the role of transactivator repression in extinction of specific target genes will be assessed by analyzing the phenotypes of hybrids that coexpress one or more cell-specific genes. Hepatoma variants with defects in expression of specific liver genes will also be isolated and characterized, and their phenotypes will be studied by complementation and transfection. Second, mechanisms of transgene extinction will be analyzed, and DNA elements involved in extinction will be defined. Hepatoma transfectants containing stably integrated, position-independent transgenes will be isolated and characterized, and expression of transgene sequences will be monitored in transfectant x fibroblast hybrids. These experiments will identify regulatory elements involved in both position- independent expression and extinction, and they should clarify the connection between chromatin structure and the extinction process. Third, the regulation of chromosomal alleles that have been specifically modified by homologous recombination will be studied. Efficient homologous modification of the human apolipoprotein B and alpha1-antitrypsin genes will be accomplished using novel, recombinationproficient chicken/human microcell hybrids, and the functions of the altered alleles will be assessed after transfer of the modified human chromosomes to expressing and non-expressing mammalian cells. These studies should help define mechanisms of eukaryotic gene control, and they may provide insights into perturbations of those controls that occur in abnormal development and neoplasia.